1. Field of the Invention
The present invention relates to a backlight unit that is a light source of a liquid crystal display device and, more particularly to a backlight unit in which a hitting noise caused by an optical sheet such as a diffusion film, reflective sheet, prism sheet, and in some case polarizing sheet installed in the backlight unit hitting an inner frame housing it is reduced.
2. Description of the Related Art
As display devices using a backlight unit as a light source, a liquid crystal television, a cell phone, and a display device mounted in a motor vehicle and the like can be cited. In a backlight unit installed in such display devices, a plural number of various kinds of optical sheets such as a diffusion sheet and a prism sheet are generally arranged forwardly of the light source such as a light emitting diode and a fluorescent tube. Because these optical sheets are arranged forwardly of the light source that generates heat when the backlight is turned on, they are subjected to the generated heat. The optical sheets may also be heated by a change in ambient temperature. When the optical sheets are heated, the widths of the optical sheets in longitudinal and lateral directions increase due to thermal expansion.
Generally, an optical sheet is housed in an inner frame formed in a part of a lower frame of the backlight unit. The inner frame is formed in view of processing tolerance of each member so that a portion for housing the optical sheet is larger than the optical sheet, in order to accommodate expansion and contraction of the optical sheet caused by changes in ambient temperature. For example, the opening portion housing the optical sheet is formed larger at least in view of the amount of size to increase by thermal expansion and in view of processing tolerance so that the optical sheet housed in the inner frame is not stretched out even when the optical sheet is thermally expanded. With this arrangement, a gap is defined between the circumferential portion of the optical sheet and the frame at normal temperature. Accordingly, the optical sheet is allowed to freely repeat expansion and contraction between a cold condition and a hot condition within the frame without suffering a restriction from the frame.
A backlight unit of a cell phone and a display device mounted in a motor vehicle also includes an optical sheet, and a supporting structure of the optical sheet is also provided with a gap defined between each side of the optical sheet and the frame, in view of thermal expansion. The optical sheet is supported so that the peripheral portion of the optical sheet is allowed to expand and contract in the gap. That is, in such backlight units, the optical sheet can freely move within the gap between the sides of the optical sheet and the frame when the optical sheet is not thermally expanded. As a result, for instance, when the display device body moves or a vibration from the outside is applied thereon, a shock may be generated to cause the optical sheet to move and hit the frame housing it, thus generating a hitting noise.
Such a hitting noise is undesirable because it may be unpleasant for a user of the display device. Therefore, it has been hoped that the optical sheet and the frame are prevented from hitting each other within the backlight unit so as to reduce the hitting noise to a level that would not be unpleasant to the user of the display unit. Particularly, it is important task in a display device installed in an auto vehicle is that of reducing direct contact between the optical sheets and the frame, caused by the vibration of the auto vehicle.
Accordingly, a projection formed integrally with an optical sheet is provided at each side of periphery of the optical sheet in a backlight unit described in JP2006-24483A, that includes an optical sheet housed freely movably within a frame by providing a gap between a periphery of the optical sheet and the frame in view of change in size of the optical sheet and the frame due to a change of ambient temperature and also of processing tolerance of the optical sheet and frame.
With the arrangement described above, possible hitting between the optical sheet and the frame occur in a small part compared with the cases where the optical sheet hits a side wall of the frame with its substantially entire side. Thus, hitting noise generated when the optical sheet moves to hit the frame can be significantly reduced than in the cases with a conventional structure
In a backlight unit described in JP2006-24483A, protrusions are formed on all four sides of an optical sheet so that a part of each protrusion hits its facing wall. Thus, a hitting noise that is generated when the optical sheet moves to hit the frame is reduced.
However, in such backlight units, in all possible directions the optical sheet can move, the position of the optical sheet within the frame depends on the entire shape of the optical sheet and the expansion coefficient of the material of the optical sheet. For example, in a case of an optical sheet having an approximately rectangular outer shape, it may need to design the sizes of the gaps between the optical sheet and the frame in the transverse direction (X-direction) and the longitudinal direction (Y-direction) of the optical sheet respectively, in view of the expansion coefficient determined by the material of the optical sheet and also in view of processing tolerance of the optical sheet. In such cases, if there is no change in size due to expansion and constriction of the optical sheet even when the projections of each side of the optical sheet hits the frame as described in JP2006-24483A, the hitting noise may be too large.